1. Field of the Invention
The invention relates to a process for operating a Coriolis mass flow rate measurement device, with at least one measurement tube through which a medium flows and which is subjected to vibration excitation; this leads to resulting vibrations of the measurement tube, a first indicator quantity being used for detection of a multiphase flow.
2. Description of Related Art
Coriolis mass flow rate measurement devices generally have a single measurement tube or a plurality of measurement tubes through which a medium flows with a mass flow rate which is to be determined. In this connection, different arrangements and geometries of measurement tubes are known. Thus, there are, for example, Coriolis mass flow rate measurement devices with a single straight measurement tube and Coriolis mass flow rate measurement devices with two curved measurement tubes.
It is common to these Coriolis mass flow rate measurement devices that the measurement tubes through which the medium flows form a mechanical vibration system which is influenced by the flowing medium. Thus, the density of the medium changes the resonant frequency of the vibration system, while the mass flow rate of the medium changes the form of the vibration. Accordingly, in addition to the mass flow rate, among other things, also the density of the flowing medium can be determined.
Coriolis mass flow rate measurement devices are characterized by high measurement accuracy. Thus, the mass flow rate can be measured with a precision of less than 0.1%. Moreover, with Coriolis mass flow rate measurement devices, in addition to the mass flow rate, other values can be determined which are derived in part from primary measurement values. Examples of these derived values are the volumetric flow rate, the mass amount and volumetric amount, and the concentrations of the flowing medium. Thus, Coriolis mass flow rate measurement devices constitute multivariable measurement devices which are often used not only for a primary measurement task, but in addition can also deliver important secondary diagnosis and quality information about a process.
Information about multiphase flows, such as two-phase flows, especially specifically detection of the occurrence of such a multiphase flow and conclusions about the form of the multiphase flow, have not be reliably available in the past using Coriolis mass flow rate measurement devices. Examples of multiphase flows, specifically of two-phase flows, are gas bubbles in a liquid which can be caused, for example, by cavitation in valves or pumps or intake of air at leaks. Furthermore, one example of a two-phase flow is a system of solids in a liquid, for example, caused by crystallization or sudden detachment of deposits in a piping system through which the medium is flowing. Finally, there is an example of two-phase flows in mixtures of insoluble liquids, therefore in emulsions, which can be caused, for example, by changing of the medium flowing through the piping system.
Conventional approaches to detection of multiphase flows in a Coriolis mass flow rate measurement device are based, for example, on determination of the drive power which is required to obtain mechanical vibrations in a Coriolis mass flow rate measurement device, such as described, for example, in German Patent Application DE 44 23 168 A1. In practice, the quantity “installation factor” has been introduced; it corresponds as a dimensionless amount to the drive power which is necessary to reach a given vibration amplitude. This quantity is designed primarily for assessment of the quality of the installation of the Coriolis mass flow rate measurement device in the piping system. However, gas bubbles in the liquid medium likewise increase the power demand and lead to an increase of the installation factor so that the installation factor can also be used for detection of a multiphase flow.
But, the disadvantage is the fact that this value alone is not sufficient for reliable detection of gas bubbles since, in particular, in addition to the quality of the installation of the Coriolis mass flow rate measurement device also, for example, the viscosity of the medium can have an effect on the power demand of the Coriolis mass flow rate measurement device. In this way, the use of the drive power for detecting a multiphase flow is associated with the danger of faulty detection of such a multiphase flow in the case of a change of the viscosity or in the case of a change of the installation conditions.
A second conventional approach to detecting or determining multiphase flows is based on the use of reference densities, such as described, for example, in U.S. Pat. No. 4,689,989. Here, the density of the multiphase flow is compared to known densities of the individual phases. However, the disadvantage is the necessity of knowing a priori the densities of the individual components of the flowing medium. Thus, this diagnosis can only be implemented in a special process and not in general.